The present invention relates to a method of manufacturing a magnesium alloy molded product, a painted structure thereof, a method of painting the same, and casings using the same.
In recent years, magnesium alloys have attracted public attention for their ease of recycling, low specific gravities, and good heat dissipating properties. As the products using magnesium alloys, the casings for household electrical appliances, such as television receivers, notebook-type personal computers, and portable minidisk players, or wheels of vehicles have been practically used. Magnesium alloys used in these products are mainly of AZ91 series for casting and AD31 series for sheet metal processing, and most of these are materials for casting, which are formed into molded products by the die-casting method or the thixo molding method.
On the surface of a metal molded product such as a magnesium alloy molded product, an anticorrosive film is normally formed for preventing the occurrence of rust, as well as providing an aesthetic appearance. Since flow marks produced during molding, or voids in which gas is trapped are present in the surface of magnesium alloy molded products, putty as well as layers of painting are applied to the surface of the molded products in order to hide flow marks and to prevent the gas trapped in voids from expanding and foaming.
For example, painting of middle to large size molded products, such as the housings of television receivers and notebook-type personal computers comprising a step (S1) of forming an anticorrosive film on the surface of a molded product; and a step (S2) of forming a painted film on the surface of the anticorrosive film. In the anticorrosive-film forming step (S1), dewaxing (S1-1) and activation (S1-2) are performed, and an anticorrosive film is formed (S1-3). Next, in the painted film forming step (S2), a primer is applied onto the surface of the anticorrosive film to form a painted film (S2-1), which is baked for curing (S2-2), then putty is applied (S2-3), the putty is baked for curing (S2-4), the putty overflown around the molded product is removed (S2-5), a primer is further applied (S2-6), and this primer is baked for curing (S2-7). Finally, top coating is applied (S2-8), and this is baked for curing (S2-9). This application of top coating is repeated twice or more as required.
In such conventional painting methods, however, a large number of process steps are required, and because of putty applying, the painting process is interrupted, and it is impossible to process the parts continuously. Therefore, there are problems that the efficiency of processing is limited, and the processing costs are high.
Therefore, the object of the present invention is to provide a painted structure of a magnesium alloy molded product having an aesthetic appearance, a method of manufacturing a molded product that can obtain a painted structure of a magnesium alloy molded product having an aesthetic appearance in a few process steps, a method of painting such a molded product, and a casing using such a molded product.
The present inventors paid attention to the fact that application of putty in a process of forming a painted film was the largest cause that complicated the coating step, that is, the number of process steps was increased and the processes could not be performed continuously. The inventors have devised to omit the application of putty, and to form the painted film by a single application of a powder paint and a single baking-for-curing treatment. As a result of studying the appearance of a magnesium alloy molded product completed in this painting process from various view points, it was known that an aesthetic appearance can be secured by setting the thickness of a painted film to a certain ratio or more relative to flow marks normally appearing in a magnesium alloy molded product, and by specifying the surface roughness of the molded product, such that flow marks can be sufficiently hidden in a single layer painted film in which application of putty is omitted.
A painted structure of a magnesium alloy molded product of the present invention comprises at least a magnesium alloy molded product having flow marks of a depth D, and a painted film of a thickness T formed on a surface of the magnesium alloy molded product, wherein the thickness of the painted film is 80% or more of the depth of the flow marks. Here, the thickness of the molded product is 0.3 mm to 5.0 mm. Also, the painted film contains no putty-filled regions.
Further, the painted structure of a magnesium alloy molded product has flow marks of which maximum depth is 100 xcexcm and maximum opening width is 0.5 mm, wherein an anticorrosive film is formed on the surface of the magnesium alloy molded product, and a painted film having an average surface roughness of 10 xcexcm or more and a thickness of 80 xcexcm or more is formed on the surface of the anticorrosive film.
According to this configuration, since the painted film having an average surface roughness of 10 xcexcm or more has been formed on the surface of the magnesium alloy molded product having flow marks as described above, light beams radiated onto the surface of the painted film can be irregularly reflected. Also, since the painted film has a thickness of 80 xcexcm or more, it is enough to hide flow marks in cooperation with the irregular reflection. Therefore, a single layer of the painted film can hide flow marks, securing an aesthetic appearance without using putty as in conventional methods. Further, the anticorrosive film prevents occurrence of rust on the molded product.
When the painted film has an average roughness of less than 10 xcexcm and a thickness of less than 80 xcexcm, flow marks may be insufficiently hidden in a magnesium alloy molded product having flow marks of which maximum depth is 100 xcexcm and of which maximum opening width is 0.5 mm.
In the painted structure according to the present invention, the painted film may be formed from a powder paint. According to this configuration, the particles of the powder paint ensure the average surface roughness of the painted film.
Also, in the painted structure according to the present invention, the painted film may contain an aluminum powder. According to this, the irregular reflection on the surface of the painted film is promoted by the aluminum powder.
Also, in the painted structure according to the present invention, the painted film may be formed from the film of the oxide of magnesium constituting the magnesium alloy molded product. According to this configuration, the magnesium oxide film prevents the occurrence of rust on the molded product, and ensured the adhesion of the painted film.
Furthermore, a method of manufacturing a magnesium alloy molded product according to the present invention comprises the steps of: melting all or a part of a magnesium-aluminum alloy containing 4 to 12% aluminum; heating a mold to which a release agent is applied to a temperature between 200xc2x0 C. and 300xc2x0 C.; injection-molding the magnesium alloy; and removing flow parts other than the product part from the obtained molded body. The thickness of the molded product is 0.3 mm to 5.0 mm.
According to this manufacturing method, a molded product that can hide flow marks with the painted film can be obtained, and such a thickness of the molded can be ensured that gives a large hiding effect.
Furthermore, the method of painting a magnesium alloy molded product according to the present invention comprises the steps of: forming an anticorrosive film on the surface of a magnesium alloy molded product; applying a powder paint on the surface of the anticorrosive film, a curing start temperature of which powder paint is 140xc2x0 C. or lower; and bake-curing the applied powder paint.
According to the above-described configuration, since the powder paint is applied, the curing-start temperature of which powder paint is 140xc2x0 C. or lower, in the step of base-curing the powder paint, the power paint is cured while it is not completely melted and remains as particles on the painted film. As a result, a painted film having a large average surface roughness can be formed. Also, since the powder paint has a low curing-start temperature, the powder paint is cured to form a painted film at a temperature lower than a temperature at which a gas trapped in voids expands and foams, so that the gas is prevented from foaming from the voids, and fine holes such as pinholes are hardly formed on the painted film. Moreover, since a painted film having a large average surface roughness can be formed, the painted film hides flow marks present on the surface of the molded product, and the appearance of the molded product can be improved without using putty. Therefore, painted structures having an aesthetic appearance can be obtained continuously, with improved treatment efficiency and at low costs.
If the curing-start temperature of the powder paint exceeds 140xc2x0 C., since the quantity of the powder paint that remains as particles decreases, the painted film having a large average surface roughness can be hardly formed. Also, since the powder paint cures at a relatively high temperature to form a painted film, it becomes difficult to prevent a gas from foaming from voids.
Furthermore, according to the method of painting the magnesium alloy molded product of the present invention, it is possible to continuously obtain painted structures having an aesthetic appearance, to improve treatment efficiency, and to realize low costs.
Also, according to the method of painting the magnesium alloy molded product of the present invention, an anticorrosive film composed of an oxide film integral with the surface of the molded product can be formed thereon.
In the method of painting the magnesium alloy molded product of the present invention, the step of forming the anticorrosive film in the above-described constitution comprises a dewaxing process, an activating process, and an oxide-film forming process. According this configuration, the surface of the molded product is cleaned and activated, and an anticorrosive film composed of an oxide film integral with the surface of the molded product is formed thereon with ease.
In the painting method of the present invention, an anticorrosive film is formed by a treatment using a phosphate in the above-described constitution. According this configuration, an anticorrosive film composed of an oxide film of magnesium formed from a phosphate and magnesium constituting the magnesium alloy molded product can be formed with ease.
Further, in the painting method of the present invention, a powder paint containing 3% by mass or more aluminum powder is applied onto the surface of the anticorrosive film in the above-described constitution. According this configuration, an enough quantity of aluminum powder is applied to the surface of the painted film to promote irregular reflection. If the quantity of aluminum powder contained in the powder paint is less than 3% by mass, the quantity of aluminum powder contained in the painted film may be too small to sufficiently promote irregular reflection.
Furthermore, a casing fabricated using the magnesium alloy molded product of the present invention has the above-described painted structure. According this configuration, flow marks are hidden, and an aesthetic appearance can be obtained.